Much progress in the electronics industry comes from circuit size reduction. This is most directly accomplished by running photolithographic processes at ever-shorter wavelengths of light. The electronics industry is currently implementing photolithographic processes employing wavelengths in the so-called “vacuum ultra-violet” (VUV). Processes using 193 nanometer (nm) light are undergoing commercialization while 157 nm wavelength light is under development as a next generation candidate.
One new development in the field is so-called immersion photolithography at 157 nm exposure wavelength as described in Switkes et al, J. Vac. Sci. Technol. B, 19 (6), 2353 6, November/December 2001; and, M. Switkes et al, “Resolution enhancement of 157-nm photolithography at 157 nm exposure wavelength by liquid immersion”, Proc. SPIE Vol. 4691, pp. 459-465 (2002). In immersion photolithography at 157 nm exposure wavelength, the optical source, the target surface, or the entire lithographic apparatus is immersed in a highly transparent high refractive index liquid. Realization of the potential benefits of this technology is dependent upon identifying liquids with exceptionally high transparency in the VUV with excellent photochemical stability, as described, for example, in M. Switkes, R. R. Kunz, M. Rothschild, R. F. Sinta, P. M. Gallagher-Wetmore, and V. J. Krukonis, “Liquids for immersion photolithography at 157 nm exposure wavelength at short wavelengths”, Proc. SPIE Vol. 5040, 690-699 (2003).
Switkes et al, Microlithography World, May 2003, pp. 4ff, disclose that 157 nm light in a high refractive index medium can simulate the effects of much shorter wavelength light for photolithographic purposes.
Considerable emphasis has been placed on identifying organic polymeric compositions suitable for use in the VUV. WO 0185811 and WO 137044 disclose fluorinated polymeric compositions having high transparency at 157 nm. Considerably less emphasis has been placed upon lower molecular weight organic liquids, which may be employed as an immersion medium in immersion photolithography in which a liquid medium is used between the projection lens of the optical stepper and the photoresist-coated substrate (typically a silicon wafer), which will receive and detect the photolithographic image. Whether polymeric or not, any material residing in the light path between the source and the target needs to be highly transparent and photochemically stable at VUV wavelengths. As disclosed in the references hereinabove cited, suitable liquids can also be used as inspection media for immersion inspection of patterned objects such as semiconductor wafers, wherein the effectively reduced wavelength will produce higher resolution imaging of small size defects.
According to Switkes et al, “Liquids for immersion photolithography at 157 nm exposure wavelength at short wavelengths,” op.cit., it is desirable that an immersion liquid layer be at least 1 mm thick for mechanical reasons and at least 95% transparent for good optical performance. A reasonable estimate of the needed absorbance for an immersion liquid is A/cm=0.22, as determined from the equationA/cm≦[log10(To/T)]/h=[log10(100%/95%)]/0.1=0.22
Where                To=light intensity in the absence of immersion fluid        T=light intensity with immersion fluid present        h=distance from lens to resist in centimetersIn general, of course, the more transparent the better.        
All known organic materials absorb to some extent at 157 nm. The issue is whether liquids can be found that are sufficiently transparent to be practical. It is known in the art that short chain hydrocarbons H(CH2)nH and short chain fluorocarbons F(CF2)nF are relatively transparent compared to their longer-chain homologues. See for example, B. A. Lombos et al, Chemical Physics Letters, 1, 42 (1967); G. Belanger et al, Chemical Physics Letters, 3(8), 649(1969); and K. Seki et al, Phys. Scripta, 41, 167(1990).
E. Albrecht et al, CERN Document Server: Preprints (2002) 1-14, ep/ep-2002-099, 16 Dec. 2002, (URL:http://documents.cern.ch/archive/electronic/cern/preprints/ep/ep-2002-099.pdf.) discloses that gas phase perfluoro-n-alkanes up to perfluorobutane are sufficiently transparent below 160 nm for use as Cherenkov detectors particularly when impurities such as oxygen, water, and hydrocarbons have been removed. However, gases exhibit very low refractive index and therefore are not suitable for use in immersion photolithography at 157 nm exposure wavelength.
Perfluoroalkane liquids are known. Perfluoro-n-pentane and perfluoro-n-hexane are available commercially at purities in excess of 99%.
Liquids having the high transparency of perfluoro-n-alkane vapors but with higher refractive index, and the desirable photochemical stability for practical use in immersion photolithography at 157 nm exposure wavelength, and other applications in the VUV are desired.